Mechanical pulverization of refrigerated plastics



S P 2, 5 R. E. BLUDEAU 2,609,150

MECHANICAL PULVERIZATION OF REFRIGERATED PLASTICS 7 Filed NOV. 5, 1949 3 Sheets-Sheet 1 SUPERHEATED NITROGEN FEED VAPOR LINE SATURATED NITROGEN 10 11 I VAPOR LINE PRECO\OLER M12 2 I V' M [8 f-LIQUID NITROGEN ITf 15 M 17 SUPERHEATED NITROGEN VAPOR LINE CYCLONE SEPARATOR 3 MI1LL CLASSIFIER PRECOOLER LIQUID NITROGEN CYCLONE SEPARATOR INVENTOR 41 ROBERT E. BLUDEAU.

I BY 42 AIRLOCK v ATTORNEY Sept.,2, 1952 R. E. BLUDEAU MECHANICAL PULVERIZATION OF REFRIGERATED PLASTICS Filed Nov. 5, 1949 3 Sheets-Sheet 3 INVENTOR U A E D U L B F. T R E B 0 R ATTORNEY by impact.

Patented Sept. 2, 1952 UNITEDASYTATIES PATENT l OFFICE MECHANICAIEPULVERIZATION F REFRIGERATED PLASTIC S 7 Robert E. Bl udeau, Denville, N. .I., assignor, by inesne assignments, to Union Carbide and Carbon Corporation, a corporation of New York Application November 5, 1949, Serial No. 125,739

This invention relates to a process and apparatus for pulverizing those thermoplastic resins which are too tough to be capable of being powdered by known processes or mills in a practicable manner. While the invention is adapted for reducing any such resin to powder, it is not economically adapted for treatment of those which are readily capable of being pulverized in the ordinary impact or other mill at less expense and without the use of the low temperature refrigerants employed here.

One characteristic of the materials for which thisinvention is especially adapted is the fact that they are good thermal insulators. :This means that in an impact mill heat of impact is developed within particles without any satisfac tory way of cooling them to prevent the heat raising the particles to softening or melting temperature. Another characteristic is a substantial degree of toughness, that is, substantial energy absorption is necessary for fracture. One example of materials for which this invention is adapted is plasticized ethyl cellulose, which is notoriously tough at low natural temperatures. Another material diflicult to reduce to powder by ordinary methods hasbeen polyethylene.

At first it was though that all that was needed to powder these and other materials was a refrigerant which was substantially colder than the brittle temperature of the material to render any of these materials brittle enough to pulverize Whether or not such were true in theory, it is not commercially economical. One reason why it is so was found to be the large amount of liquid refrigerant like liquid air or liquid nitrogen that was required and alsothe time necessary to soak even reasonably small particles of the plastics to extract heat within the material below its surface. Several unsuccessful attempts had been made to immerse polyethylene inliquid nitrogen before pulverizing it for the purpose of embrittling it. The material did not apparently get thoroughly chilled well below the surfaces of the material. Also on removal from the liquid refrigerant the surface of the particles absorbed heat en route to theomill; Another difficulty was found to be in the impact mill functioning in a manner similar to a suction pump, sucking in a quantity of air at room temperature to reduce the chilling effect of the liquid refrigerant. Z

An object of this invention is to provide an efiicient method of and apparatus for powdering those thermoplastics which are tough and difficult to powder in conventional ways. More 10 Claims. (Cl..241):

specifically an object is to pulverize those more difiicult materials, such as polyethylene and ethyl cellulose, in an efficient manner. A further ob-' ject is to provide an enclosed system in which the difficult materialsrto pulverize may be cooled without circulating ambient air into a mill in objectionable quantity. Another object is to provide a method for treating such materials by chilling at leastv the outer portions of each particleimmediately beforeentering an impact mill so that at 'leastsurface portions of the materials are chilled well below their brittle points. Yet a further object is to effect such cooling and breaking up by impact in alternate operations in order to increase the fineness of material that is pulverized. Another object is to carry out the aforesaid impact pulverization under at least slight superatmospheric pressure so that no warm air is drawn in with the material supply. Still a further object is to promote eflicient operation of this process and apparatus by precooling the material fed to the pulverizing mill with the aid of vaporized refrigerant moving either countercurrent or in parallel flow to the suppliedmateriala 1 According to this-invention, the foregoing disadvantages have been overcome and an efficient manner of chilling and pulverizing troublesome materials like polyethylene, ethyl-cellulose, and others has been discovered. Yet another view ofthe present invention may be said to reside in the discovery of what objections and difliculties were possessed by the various prior attempts to accomplish the same results. More specifically, this invention contemplates chilling the particles of plastic immediately before they are fed to an impact mill, whereby at least some surfaceportions of the particles are chilled to well below their brittle temperature. This chilling is obtainedby spraying precooled material with a liquidrefrigerant such as liquid nitrogen immediately, before the material is subjected to impact under slightly super-atmospheric pressureand the vaporized refrigerant is passed in contact with the incoming material for precooling it. The escaping nitrogen serves to prevent admission to the mill of atmospheric air at room tem-' peratures. At least the brittle exterior of the chilled pieces is thought to break up into smaller particles before their temperature has risen too high in an impactmill.

.' Figyl illustrates a' single stage apparatus for carrying out the present invention. Fig; '2 is a diagrammatic representation of a multi-stage process and apparatus used in this invention.

Fig. 3 is a chart indicating the percentage of fines of different sizes at the conclusion of each stage of a three-stage process when powdering polyethylene. Fig. 4 illustrates another type of apparatus for precooling and powdering the materials. Fig. 5 shows a somewhat similar apparatus to that of ,Fig. 4, the chief difference residing in the fact that the liquid refrigerant is sprayed on the material in a lower instead of in an upper port-ion of a heat exchanger.

One material treated is a polyethylenehaving a molecular weight of 20,000 and a melting 'temperature of between 105 C. and 110 'C. This material was sheared into lengths-of about from rods having a diameter of :about 1% and placed in the hopper [0 so that it is fed by the preliminary feed screw H, driven by'the motor [2 to a main feed screw l3 driven by the motor [4. The material is moved from the right to the left in the drawing of Fig. 1 and then falls down the passage it .onto the final feed .screw l fi, driven :by the motor fl I. .Just before the material 'is 'fed into a tmill ifor breaking it up, it is sprayed with lliquid'itnitrogen, the spray being indicated by'the mumera'l' 1 8. The impact mums rotates at a rapid rate. .Aiter passing through the min the material fallsiin'to receiver 20. The vaporized refrigerant or gaseous nitrogen passes through the line 321 "to the left end of the main feed screw l3 where-zit passes in -.contact with the material beingtfed 'in ."for reducing its temperature; :From'the rightend' of the screw conveyor 13 thevaporized'zrefrigerant is-drawn ofi through the. passage" 1'22 'to :a jacket-.13 around the hopper'ilD-jr;

' 'The impact mill used had a diameter of about 8."., and a speed of 10,0001? PJM. It developed at'linear velocity 'iinthe particles of substantially 22 ,500 ftliper minute. The lower molecular weight polyethylene materials :having lower softening temperatures are easier topowder because their impact strength 'Iis less than is that ofthe ma terials having the higher -molecular weight and melting temperature. The microscopic crystals of polyethylene are believed to be of the order of a micron 1or va amillionthof a 'meter in size. Its crystalline phase exists irom' below its brittle point .up to withina' few degrees of its melting temperature. The impact strength of polystyren'e is .26 :to .6 foot pound per inch of notch [21inch x /2- inch'notched barlzod test. The impactstrength of ethyl-cellulose is Z. to 11.5, and that vfor the vinyl chloride resin is .3 to 1.'0=in the same units. The-impact-streng th of polyethylene is from'3 to ashigh as wit-pounds per inch of notch inch ai inch notched bar Izod test. The thermal conductivity For polyethylene is 6 to 8 10- calories per square centimeter per second per degree Centigrade per centimeter in thickness. The polyvinylidene chloride-known as Saran has a thermal-conductivecharacter of only '2.2 -l'0 calories persquare centimeter per second per degree Centigrade per centimeter in thicknessl Vinyl chloride (plasticized) has a thermal conductivity 01 '319 to 4.0 in the same units. l Polystyrene has a thermal conductivity of 1:8 to 220.; and ethyl cellulose 4. to 7. thermal conductivity, each in itheisame aforementioned units. The other polycrystalline plastics and other tough thermoplastics have impact-strengths and thermal insulating properties between the 55% of the rmaterial passed through a I l-mesh screen. Smaller percentages, as indicated in this diagram, passed through the screens of the mesh extremes mentioned. 'Ihe polyethylene has the size-indicated in the. abscissa.

As shown inFi-g. 2, the powdering process for polyethylene is adapted to be carried out more efliciently in a multi-stage operation where greater fineness is desired in the powdered material. The material is fed into the hopper l0 and passes through the main feed screw 43 toan impact m'ill I9. "Liquid itrogen is supplied to the material just before it is fed into the impact mill in the manner described inconnection with Fig. 1. The vaporized'refrigerantand powdered material from the mill 4% are discharged through the passage 26 onto theclassifierzl. The fines of appropriate size are discharged tlirough'the' passage 2-3. In order to prevent warm air getting :into thecold material, the pulverized product from the passage 28 is discharged through an air lock 29, that is, batches of the'mater'ial are discharged without having any "substantial amount of the coldgas leak'cu't'or an undue quantity'of warm air get in. "The solidmaterial-andvaporized refrigerant from' the first stage then passes "by the line 30 to "the mill 31 of the second stage. Here again, j-ustzbe'fore the material. is fed into the mill 3|, liquid nitrogen is sprayed on it. Fronilthe =mi-ll 31 the Napor'ized refrigerant rises through the line '32 and enters the cyclone separator 3 in which any-solid particles are separated outv and 'passed bythe line 35 back into the classi fier :21 for removal. 'I he vaporized refrigerant is discharged from the separator 33 through the Pipe fill-and enters the main feed screw 13 for precooling '"thematerial' before it is supplied to the rfirst stage mill. The solid material discharged trom the mil1r'3l passes 'throughthe passage 36 to :asecond classifier '37, from which the powdered material of appropriate size is drawn .off through theline '38 and air lock 39 in a :manner already described in connection with the -.drawing'off of material from the first'stage. The material for further pulveriz-ation passes from the classifier 13.! through the line 40 into the .mill llil :where the material is again sprayed by liquid nitrogen just before its entry into the mill M. .After passage through this mill the material discharges through the airlock 42. The vaporized -z-'efrigerant from the mill 4| passes through the line 43 into the separator 44, the gaseous material being discharged through the line '45, which empties into the line 34 for feeding into-the precooler i-3f Solid particles of material' remove'd from the separator 34 passiby the line 46 into the classifier 31, Where they can be discharged through the line 38 as described. In order ito prevent particles of excessive size being discharged from any impact mill, a screen having openings of A" in size is located on the outlet of theifirst stagelmill ll. The screen on the second mill 3! has openings of in size, and on the third mill 4| the openings are 1 5" in size. Particles 0f :material'l'arger than those capable of being discharged are thus keptin the'mill until brokenup. I s i The center and right portions of the Fig. 3 diagram illustrate how much finer material is obtained from the second and third stages than is the'casein the first stage. r

' lEfolyethylene is the only material found to need multistage treatment. The other materials mentioned including ethyl cellulose, vinyl chloride (plasticized), vinylidene chloride, at vinylideneacrylonitrile co'polymer, and polystyrene: are all adapted for satisfactory pulverization in aIsinf gle stage process with liquid nitrogen. 1 Wher ever thetenn ethyl cellulose is used herein, a'pl'asticized material is referred to. With ethyl' cellulose granules about A of an inch in size three. separate runs were made'of at least two hours duration each. Mill screens of 14; inch round and "1 3' inch herringbone openings were used with no apparent difference in the grinding rate or liquid nitrogen consumption. Approxi mately 95% of the powdered material from the mill passed through a -mesh screen. The averg'rindingratewas 135 pounds. per hour ate horse power. I The consumption of liquid nitrogen averaged 5 pounds (6.9 cubic-feet) .per pound of ethyl cellulose pulverized.

The results from powdering batches .of polystyrene are as follows:

' I Au Test No. 1 Round Screen Polystyrene Ground 1h 42 Liquid Nitrogen Consumed. lb; 37 Liquid Nitrogen requiredper lb. Product. lb" 0. 88 Time of Test "171111.; Product Grinding Rate "lb. per hr 100 Grinding PowerRequired "H. 5 Mill Motor Speed "R. P. I 9, 500

Screen Analysis Mill (Rotap) (36lb.) l p separatef l Percent On 20 O. 3 Through 20 On 35.-.. 12.5 I Thmugh 8 it it; ttsantia 3'3 4 Through 2 n 31. 0' Through On 80 34. 4 7. 5 Through 80 On 100; 18. 3 6.5 Through l00 36.5

100.0 Total 100.?)

364 Test No. 2 Round" Screerr:

Polystyrene Ground. "a. 32 Liquid Nitrogen Consumed... 84 Liquid Nitrogen Required per lb. Product 2. 62 Time of Test mm.. 33 Product Ggindingfiltate. (.1 5g Grinding ower equire Mill Speed .R. P. M 9, 500

' Min Cyclone Screen Analysis (Rotap) (28 lb Separator ,(4 1b Percent Percent Tests on powdering a polyvinylidene chloride known 'as Sarah were tried using two sizes of mill screens, one of an inch diameter openings and one .039 of an inch in diameter. The

smaller screen openings required higher nitrogen consumption per pound at an appreciably lower grinding rate. A screen analysis'is as follows;

j Virgin Size Screen Mill clone Size Screen Saran Per- Per ce'nt cent Percent On 42 0. 4 0.1 On 20 0.00 Through 42011 50.-.. f 0.6 0. 5 Through 20 0n-100 37.0 Through 50 On 15.9 6.6 Through On 200. 15.2 Through 80 On ML. -57. 6 1 Through 200 i. Through 100 25.5

Total 100.0

- A summary of follower H Suntfizarg jof tests with doublectttls' mn scrap:

the tests on pulverizing Saran Lb. Nitro- Product Product Screen- Tune 7 Run No. 55%? Ground, Rate, Opening, vOf Run, Pulverized b' Mm 1.44 66 -80.7 I at. 1'40 While toughness strictly w depends on fthe amount of energy a material will 'absorb w-ithout breaking when stiff, it is diflicult of precise measurement. Impact stren'gthfis a rough "but ready indication of'toughnes's. Inasmuc'h as the impact strength of ethyl cellulose and of polyethylene are each high, it isbelieved' that this process and apparatus are adapted for pulverizing any resinous plastic itself that is not toughenedand strengthened by fabric or fibrous reinforcements. This ViW is confirmedby the properties fcfhart forming a part of the 1947 edition of Modern Plastics encyclopedia; which shows that no otherf-resi'nous plastic'byfitself and not strengthened by a fibrous-or fabriore inforcernent has an impactstrengthhigher than that of ethyl cellulose; Polyethy1ene-wasfound to'be the most olifiicult material to powder, due not-only to its high impact strength or toughness'but'due also to'its large percent elongation at ultimate strength in tension. Any operation depending on shredding 'or p'iilling' apart polyethylene results in elongated fibrous appearing particles which'are not as finely divided 'as is usually desired. All the materials tried and mentioned herein. have a thermal insulating character which when the material .is tough and placed in an impact mill makes the heat of impact difficult-to remove before particles coalesce.

The'material to be powderedis placed in the hopper 48 from which it is fed by the conveyor 49 into the heat exchanger 50, in. Fig. 4. Liquefied gaseous refrigerant is sprayed from the nozzle 5i onto the material. Thedrawing shows the upper shelf: in the exchanger 50 is provided with 'a central aperture and a rotating arm directs material inwardly .to pass through said aperture. The next lower shelf has an opening on its radially outer portion toward which the material is. directed by a rotating arm; The succeeding shelves are arranged so thatthe material and vaporized refrigerantt'ravel inwardly over'one and outwardly over'the-next, whereby posite-directions over any two adjacent shelves:

per.

Erom'the heat exchanger 50. the material moves by gravity into the-impact mill 52, which is of the .same type as the mill in Fig. 1. An air lock 53 enables the fine material to be drawn off without substantial loss of cold gas. The vaporized refrigerant at slightly superatmospheric pressure passes from the mill through the pipe 54 into the hopper. A 'baiile '55 directs the cold gas downward through the material in the hop- Some of the vaporized; refrigerant travels from the heat exchanger il-throug'h the conveyor 4 8 for cooling material in the conveyor.-

In Fig. 5 the apparatus isquite similar to that in Fig. 4. The heat exchanger '56 has the nozzle 51 for liquefied refrigerant located in the lower portion whereby the refrigerant gas moves counter-current to the travel of the material. 'As before, some of the vaporized refrigerant moves through the conveyor 49 while the major portion of it enters the mill 52 by way of the duct 59. A conveyor 58 is desirable in the base of the exchanger 56. v

The apparatus of either Fig.4 or 'Fig. 5 is capable of being used in series for very'fine pulverization. Having the spray close to the mill aids in cooling the mill. 7

Under the process of this invention at least the outside of particles of material are chilled. With thermal insulating materials like polyethylene, for example, the outside of the particles is believed to be placed under slight tension when chilled, which facilitates the rupture of at least the chilled outerportions. An advantage of this invention resides in the shapeof the powdered particlesf-rom the mill. These .areusually sharp pointed and angular, without elongated fibrous .appearing'projections.

By the term liquefied gas is meant a refrigerant which is gaseous at room temperature.

What .is claimed is: I I

'1. The method of powdering pieces of tough synthetic thermoplastic resins having a thermal conductivity of as little as (1.8 to 8) X10 calories per square centimeter per second per degree Centigrade vpercentimeter of thickness, having .26 to ,10 foot pounds per inch ofnotch /2 inch x 5 inch bar Izod impact strength, and a percent elongation at ultimate strength in tension of 1.5 to 600, said method comprising chillingprecooled surface portions of said pieces of resin to a temperature well below its brittle temperature by a spray of liquid refrigerant having a temperature adjacent that of liquid nitrogen, and substantially immediately thereaftersuddenly increasing the'velocity of said pieces of resin by subjecting said pieces to impact of a magnitude capable of imparting alinear velocity .to'ithem of at least several thousand feet per minute, whereby at least surface particles are broken off from said chilled pieces.

'2. A process of chilling and pulverizing by impact a tough thermoplastic resin having a high impact strength of between'2 and 11.5 foot pounds per inch of notch /2 inch x /2 inch notched bar Izod test, said process comprising contacting pieces of the material with a refrigerant which is a "liquefied :gas having a temperature adjacent that of'liquid nitrogen, which has been vaporized and isatslightly superatmospheric pressure while feedingthe-sameto an impact mill, spraying said refrigerant in liquid phase onto said material closely adjacent .said mill and after it has been cooled .by said vaporized refrigerant to chill at leastouterportions of-saidmaterial to Wellbelow its .brittle point, subjecting the material to impacts in'zsaid mill, separating particles below a 8 predeterminedv size .from those above said size, and precooling the material fed to said mill .by said contact with vaporized refrigerant from said milll- I 3. .'Ihe method .of powdering pieces of tough synthetic thermoplastic resinous material having thermal conductivity of (1.8 to ,8) l-0- calories per square centimeter per second per degree-Centigrade per centimeter of thickness, having impact strength of .26 :to 11.5 foot pounds perinch of notch /2 inch x /2 inch bar Izo,d, and apercent elongation at ultimate strength in tension of 1.5 to 600, said method comprising precooling pieces of said material under slightly .superatmospheric pressure, moving the precooled particles of material under said pressure toward .a mill for breaking said material into smaller, particles just before reaching said mill, chillingiprecooled surface portions. .of .said pieces of material to a temperature Well below its brittle temperature by aspray of liquid refrigerant hayinga temperature adjacent that of liquid nitrogen, substantially immediately thereafter subjecting said pieces .of mate rial to impactior breaking .off .at least surface particles from said chilled pieces, vaporizingliquid refrigerant, from said spray, and/returning vaporized refrigerant from said chilling and from .said mill for said precooling.

4. A method according to claim 3 in whichsaid precooled thermoplastic resin is moved through said spray after having been moved toward said mill and again contacting said resin and refrigerant after the resin has moved through said spray.

5. A method according to claim 3 in whichisaid thermoplastic is polyethylene and the impact milling and spraying with said liquid refrigerant is performed also in a subsequent stage upon pieces of saidresin above a desired size.

6. .A method according toclaim .3 in whichsaid resin is polyethylene.

7. A method according to claim 3 in which said material is from a, classconsisting of polyethylene, ethyl cellulose, vinyl chloride, polystyrene, and polyvinylidene chloride.

8. An apparatus for pulverizing tough thermoplastic resinous material comprising an enclosed impact mill, a'conveyor for moving pieces of the material toward said mill, an enclosure for said conveyor leading to and connected with saidmill, a spray head for liquid refrigerant between said mill and conveyor and closely adjacent said mill and within said enclosure, means for operating said mill, a return passage for vaporized refrigerant from said mill at least partially'within said enclosure around the conveyor for retaining the vaporized refrigerant and passing it through-the material 'for precooling said material under slightly superatmospheric pressure to prevent access of air at room temperature to the mill and material being precooled, and .an air lock by means of which pulverized material from said mill may be withdrawn Without substantial loss of cold gas from said mill or the admission of substantial air at room temperature.

9. An apparatus according to claim 8 in which said spray head is within an upstanding heat exchanger between said conveyor and mill.

10. Apparatus for pulverizing plastics comprising an impact type pulverizing mill, a conveyor for moving material toward said mill, a spray head close beside the mill for directing liquid onto particles of material moving toward said mill, at least a substantial portion :of liquid from said head being directed transversely of the moving 9 material at an acuteangle thereto adjacent said mill, a supply source 'of' liquefied gas refrigerant having a temperature adjacent that of liquid nitrogen connected to said spray head, an enclosure for said conveyor, spray head, and mill, and

a return passage from said mill for vaporized" REFERENCES CITED The following references are of record in the file of this patent: v

UNITED STATES PATENTS Name Date. Ager Jan. 1, 1884 Hutchinson Nov. 21, 1899 Number Number Number 10 Name Date Phelp Mar. 15, 1910 Rodgers Dec. 27, 1921 Diepshlag May 15, 1923 Loizillon Oct. 9, 1928 Ruprecht Apr. 30, 1929 Sinclair Jan. 28, 1930 Hurt Oct. 23, 1934 Noel Dec. 1, 1936 Williams July 20, 1937 Jones Mar. 4, 1941 Huddle June 16, 1942 Washburn Nov. 10, 1942 Sheldon Apr. 6, 1943 Cuno Apr. 25, 1944 Brackett Sept. 6, 1949 Knight Apr. 25, 1950 FOREIGN PATENTS Country Date Germany Dec. 27, 1909 

2. A PROCESS OF CHILLING AND PULVERIZING BY IMPACT A TOUGH THERMOPLASTIC RESIN HAVING A HIGH IMPACT STRENGTH OF BETWEEN 2 AND 11.5 FOOT POUNDS PER INCH OF NOTCH 1/2 INCH 1/2 INCH NOTCHED BAR IZOD TEST, SAID PROCESS COMPRISING CONTACTING PIECES OF THE MATERIAL WITH A REFRIGERANT WHICH IS A LIQUEFIED GAS HAVING A TEMPERATURE ADJACENT THAT OF LIQUID NITROGEN, WHICH HAS BEEN VAPORIZED AND IS AT SLIGHTLY SUPERATMOSPHERIC PRESSURE WHILE FEEDING THE SAME TO AN IMPACT MILL, SPRAYING SAID REFRIGERANT IN LIQUID PHASE ONTO SAID MATERIAL CLOSELY ADJACENT SAID MILL AND AFTER IT HAS BEEN COOLED BY SAID VAPORIZED REFRIGERANT TO CHILL AT LEAST OUTER PORTIONS OF SAID MATERIAL TO WELL BELOW ITS BRITTLE POINT, SUBJECTING THE MATERIAL TO IMPACTS IN SAID MILL, SEPARATING PARTICLES BELOW A PREDETERMINED SIZE FROM THOSE ABOVE SAID SIZE, AND PRECOOLING THE MATERIAL FED TO SAID MILL BY SAID CONTACT WITH VAPORIZED REFRIGERANT FROM SAID MILL. 